The invention relates to a stereo decoder for a multiplex signal comprising a demultiplexer having a signal input and a carrier input, the signal input being coupled to an input of the stereo decoder via a pilot-elimination circuit for reducing a pilot signal of a multiplex signal, the decoder also comprising a phase-locked loop having a phase-locked oscillator and a phase detector, said phase-locked oscillator being connected to the carrier input of the demultiplexer and the phase detector being provided with a reference input stage.
Such a stereo decoder is used in the tuner-amplifier unit ST 8080 of Matsushita Electric Trading Co. Ltd. Japan and published in print in the manual thereof.
Some sound recording devices, such as, for example, tape recording devices utilize automatic volume control, wherein the volume of the sound signal to be recorded is measured and brought to a certain level before actual recording occurs. If such sound recording devices are used for recording the sound signal at the outputs of a stereo decoder, the pilot signal (normally 19 KHz) might be included in the measurement of the amplitude of the sound signal, thus preventing a correct volume control and an optimal recording. To avoid this, the amplitude of the pilot signal of the multiplex signal which is applied to the demultiplexer is reduced by means of the pilot elimination circuit.
The carrier required for decoding the multiplex signal is produced in the phase-locked oscillator. The phase thereof is compared with the phase of the pilot signal of the multiplex signal by multiplying, in the phase detector, the oscillator signal by the multiplex signal applied to the phase detector via the input of the stereo decoder. The output signal of the phase detector being the control signal of the phase-locked oscillator therefore comprises, in addition to a desired component which indicates the phase relation between the oscillator signal and the pilot signal, unwanted components which originate, for example, from the sound components of the multiplex signal. These unwanted components may influence the frequency of the phase-locked oscillator so that a proper phase locking of the carrier to the pilot signal is disturbed. An optimal decoding of the stereo signal from the multiplex signal is then impossible.